Gaseous-solid state power limiter



Feb. 13, 1968 D. c. BRoDl-:RlcK 3,369,196

GASEOUS-SOLID STATE POWER LIMITER Filed May 5, 1966 2 Sheets-Sheet lFeb.l3,l968

Filed May D C-BRODENCK GASEOUS-SOLID STATE POWER I JUVITER JRS 2Sheets-Sheet 2 United States Patent C "f 3,369,196 GASEUS-SOHD STATEPOWER LIMITER David C. Broderick, Beverly, Mass., assignor to Metcom,Inc., Salem, Mass., a corporation of Delaware Continuation-in-part ofapplication Ser. No. 214,546,

Aug. 3, 1962. This application May 3, 1966, Ser.

2 Claims. (Cl. S33-13) This application is a continuation-in-part of acopending application Aug. 3, 1962, titled Gaseous-Solid State PowerLimiter, David C. Broderick, inventor, Serial Number 214,546, now PatentNo. 3,249,899 issued May 3, 1966.

The present invention relates to devices for limiting passage of peakloads of electromagnetic energy; more particularly, it relates todevices for limiting the propogation of destructively high power radiofrequency signals through coaxial wave guides.

. Radar, microwave systems, and other high frequency systems requirethat sensitive system components, receivers, and crystal detectors beprotected from direct incidence of high powered radio frequency energypulses. For instance, in radar equipment exposure of sensitive parts ofthe system to destructively high powered microwave pulse signals ariseswhen the high powered pulse generator, most frequently a magnetron,tires, or when directly beamed signals from a second microwave systemfall on the antenna and are transmitted back to the receiver or othersensitive elements of the system. When protection from excessively highpower pulse signals is being designed, for example for use in a radarsystem, it is known what frequency signals will be emitted by themagnetron or signal generator incorporated within the system. However,it is not predictable what frequency range of signals arising from otherradar sets may be beamed directly onto the antenna of the system, It is,therefore, important that the sensitive elements of a radar system beprotected against stray high powered radiation throughout a wide rangeof frequency bands.

Gaseous electron discharge tubes which have a resonant discharge gapmounted within a cavity have long been utilized as TR ortransmit-receive tubes, and as ATR or anti-transmit-receive tubes, inradar systems. In typical installations, the TR tube is mounted so thatupon incidence of a high power pulse signal, the tube res, that is, thegas within the tube cavity ionizes and electrons are discharged. As aresult of the electron discharge, a highly conductive electron stream orarc shorts the wave guide, which in the conventional installationdesigns isolates the receiver from the full amplitude of the incidenthigh powered signal.

One signicant disadvantage of the conventional gaseous TR tube when usedalone as the sole protective element for a system receiver is the factthat in the brief interval of time between the incidence of the signaland the actual shorting of the TR tube by the electron discharge, asubstantial power spike passes the tube and travels into the receiver.This initial power spike passes the gap even though a powered Keep-aliveelectrode is mounted in the resonant gap of the electron discharge tube.

A continuing trend of utilizing more powerful signal generators and evenmore sensitive receivers has heightened the need for improved, morereliable protection of receivers and particularly receiver crystals inradar systems.

The ideal microwave receiver protective device affords substantialattenuation of high powered signals throughout the dynamic range ofsystem frequencies, a low insertion loss for low powered signals,suiiciently rapid re- 3,369,196 Patented Feb. 13, 1968 spouse time tominimize or even eliminate the initial spike from reaching the receiver,a rapid recovery time to accommodate high pulse repetition rate, anddependable long life operation. Some solid state semiconductor devicesexhibit in a general way the highly nonlinear power attenuationcharacteristics that are required to pass low signals with minimuminsertion loss and attenuate high power signals with high efficiency.However, none of the existent solid state components is suitable fordirect substitution for the gaseous electron discharge tube. The reasonfor this unsuitability in existing solid state devices for directsubstitution for gaseous electron discharge tubes arises from the factthat these semiconductor devices are limited by-their maximum powerdensity; that is, most solid state or semiconductor devices areextremely small and dissipate only a limited amount of energy byradiation or convective cooling. Another limitation of existingsemiconductor devices which renders them unsuitable for directsubstitution for gaseous electron discharge TR tubes is that theyexhibit relatively large reactances which cause reection of incidentpower. High frequency performance in solid state devices, that is, inthe megacycle and kilomegacycle region, requires that the distancesbetweenthe electrodes and boundaries of the component parts of thedevice be sufficiently short so that the junction reactance, spreadingresistance and transit times of electrons or holes through thesemiconductor, will be consistent with the high frequency requirementsof the device. In general, then, the smaller the solid state orsemiconductor structures, the higher the frequency response beforecut-off exhibited by the device but the less capacity in general thesemiconductor device will exhibit to dissipate heat in higher powerapplications and, in a general sense, the less easily will the device bematched in impedance to the input and output structures with which it ismounted in a given system.

There exists, then, need for improved power limiting devices whicheffectively function across a wide frequency range and aiford lowinsertion loss for low power signals and high attenuation for high powersignals. Presently used gaseous electron discharge tubes have typicalinsertion losses of 0.5 db of low power signals, that is, signalstrengths of 1-2 watts peak power, and 10 db attenuation of high powersignals, that is, signal strengths above 10 watts peak power, through afrequency band pass range of ten percent.

The copending application previously referred to contained aspecification for essentially stripline devices. As herein will be morefully described, devices with similar functions may be provided forcoaxial wave guides.

Accordingly, one object of my invention is to provide a novel powerlimiter for attenuating excessively high power signals ranging from UHFfrequencies upward, for use in coaxial wave guide system.

Another object of my invention is to provide a highly efficient radiofrequency power limiter which reduces and substantially eliminates theinitial power spike which passes conventional gaseous electron dischargetubes, for use in a system utilizing coaxial wave guides.

Another object of my invention is to provide a broad radio frequencyband power limiter utilizing lightweight, compact, solid statesemiconductor structures, for use in a coaxial wave guide member.

Still another object of my invention is to provide a novel, ruggedgeneral purpose radio frequency power limiter.

These and other objects of my invention will be apparent from thefollowing drawings, specifications, and claims.

bodiment of my invention.

FIGURE 2 is a longitudinal cross-section view along the plane 2 2 of theembodiment of my invention shown in FIGURE l.

FIGURE 3 is a transverse cross-section view as seen at plan 3 3 of theembodiment of my invention shown in FIGURE 1.

FIGURE 4 is an enlarged fragmentary view of the embodiment of myinvention shown in FIGURE 1.

FIGURE 5 is a schematic circuit diagram of the embodiment of myinvention shown in FIGURE 1.

FIGURE 6 is a fragmentary View of a variation of the embodiment of myinvention shown in FIGURE 1.

FIGURE 7 is a schematic circuit diagram of Athe variation of theembodiment of my invention illustrated in FIGURE 6.

FIGURE 1 is an exterior view of a preferred embodiment of my inventionwhich is comprised of a length of coaxial conductor 10 having an innerconductor 12 and an outer conductor 14; a second length of coaxialconductor 16 of a different and, inthe present embodiment, smallerdiameter is mounted to the output end of conductor 10. My power limitingdevice is mounted to a radar system, for instance, by means of acoupling flange 18 at the input end and coupling flange 20 at the outputend. The joining at 22 of the two coaxial wave guide sections 10 and 16having different diameters permits adjustment of the impedance tofacilitate maintaining the desired impedance of the device as a whole.The effects of the various sections as illustrated are discussed below.

A tube mounting structure 24 which houses a twoterminal electrondischarge tube is positioned on the side of the lcoaxial wave guide 10.Similarly, mounting structures 26, 28 and 30 for housing solid statenonlinear components are fixed to the side of the smaller coaxial waveguide section 16. The arrangement of the solid state housing structures,is best seen in FIGURE 3.

The lcoaxial wave guide 10 is provided with two shunt reactive posts 38and 40 mounted in electrically spaced relationship. The drawingsillustrate a spacing between the shunt reactive posts of one-halfelectrical wavelength; however, any even integral number of quarterwavelengths separation would serve equally as well. A standing wave ispropagated in the electrical cavity formed within the space between theposts 38 and 40. The Q of the cavity is determined by the diameter ofthe posts.

Added rigidity is obtained in the coaxial wave guide by the presence ofthe posts 38 and 40. Similarly, a support ring 42 is inserted betweenthe inner and outer conductors at the wave guide discontinuity betweensections 10 and 16. The discontinuity in wave guide size is not requiredto form the electrical cavityf The smaller size of the Wave guidesection 16 is merely more convenient for mounting the solid statecomponents described below and facilitate impedance matching within thelimiter to compensate for the presence of the active nonlinear elementswhich comprise the limiter combination.

A gaseous electron discharge tube 46 is mounted, as will be describedbelow, at a position midway between the posts 38 and 40 at a point ofmaximum amplitude of the aforesaid standing wave pattern. The tube 46 iscomprised of a rst terminal 48, a ground terminal 50, an ionization gap,the sides of which are connected respectively to the terminals 48 and50, a quantity of ionizing gas mixture sealed within an envelope 52 anda keep alive electrode. The ionization gap and keep alive electrode arenot shown in the illustrations, but are conventional in all respects.The keep alive electrode is powered through I the keep alive terminal54. v

A threaded boss 56 is integrally mounted to the outer conductor 14. Acentral aperture is provided in the boss 56 and is aligned with atransverse aperture which extends diametrically through the coaxial waveguide as may be seen in the figures. Dielectric spacers 58 and 60 areinserted within the inner conductor to provide rigidity and to provideadded support fora socket contact 62. The

tube terminal 48 is inserted into and held by the socket contact 62 thusmaking RF contact with the inner conductor. A threaded cap 64, which isprovided with an internal flange 66, attaches to the boss 56. The ilange66 bears upon the ground terminal 50 of the tube providing bothelectrical ground contact and mechanical clamping action which holds thetube in place.

A tuning slug 70 is suspended in the wave guide aperture opposite thetube 46. The slug 70 is mounted through a dielectric pad 72 andsuspended from a tuning slug cover 74, which permits small movement ofthe tuning slug by means of a threaded bolt. Adjustment of the positionof the tuning slug enables the operator to make ne tuning adjustmentsafter replacement or remounting of a tube.

Two terminal semiconductor devices 80, 82, 84, described more fullybelow, are mounted as will appear from the following description betweenthe inner and outer coaxial conductors 12, 14 within the housings 26, 28and 30 respectively. The nonlinear characteristics of the semiconductordevices, for example, are such that they are essentially reactive inresponse to low power incident RF waves and essentially conductive inresponse to high power incident waves. In an equivalent circuit thesemiconductor would appear as highly capacitive in response to low powersignals and appear conductive in response to high power signals.

The shunt inductor posts 38 and 40 in an equivalent two wiretransmission system would be equivalent to two parallel inductancesshunted between the two transmission wires. The inductance of the posts38 and 40 together with the capacitance of the semiconductor devicesform a band pass filter S6 for signals passed by the wave guide.

Silicon diffused junction diodes of the mesa design, commonly calledveractors, have operated satisfactorily in speci-fic embodiments of thisinvention. A typical diode, such as was used in operating models of thisinvention would |be conductive in the forward path at 0.6 volts, withcutoff frequency above 10() kmc., dissipating up to 300 milliwatts ofpower. However, numerous other diodes and two terminal semiconductordevices are available which will serve equally well.

The limiter section 16 of my device is comprised of a coaxial wave guideof reduced diameter having inner conductor and outer conductor 92. Theouter conductor 92 has a heavier wall than conventional in order tomount the structures described below. The semiconductor device or diode80, for example, is mounted within the housing 26 which is comprised ofla threaded boss 94 mounted integrally to the wave guide outer conductor92. The boss 94 is provided with a central aperture which is alignedwith an aperture 96 cut transversely through the outer conductor 92.

A socket contact 98 is mounted by solder or brazing to the innerconductor in alignment with aperture 96. The diode 80, which has twoshort terminals designated as 80a and 80h, is positioned so thatterminal 80a is inserted into the socket 98 and makes electrical contactwith the inner conductor 90. The diode contact 80b is then within thecentral aperture of the boss 94. An enlarged diode terminal cap 100 istted over the diode terminal 80h.

A anged dielectric sleeve 102 fits` over the body of the diode 801andboth supports and isolates it from the outer conductor 92 at the pointwhere the diode passes through the outer conductor aperture 96. Adielectric spacer 104 is placed over the end of the terminal cap 100. Anongrounded conlducti-ng ring 106 is litted about the dielectric sleeve102 to provide heat conduction away from the diode. A metallic groundedheat sink is juxtaposed to the dielectric spacer 104, and the wholeassembly is held together by means of a threaded cap 110. The mountingarrangement of theabove described parts is most easily seen in FIGURE 4.

In the embodiment of my invention illustrated in FIG- URES l through 4,a bias circuit 114 is provided which serves to couple a small quantityof RF energy from the Wave guide and bias the diode to conductionsimultaneous with or prior to the arrival of high energy RF waves at theplane of the diode. The bias circuit 114 is cornprised of a probe 116grounded at one end 118 and connected to the diode terminal cap 100through a series rectifying diode 120 and a small inductance 122 forphase adjusting. The bias circuit is shown as being supported on twoinsulated support posts 124, 126. A recess 128 and small connectingopenings lare provided in the outer conductor 92 and through the side ofthe boss 94 to accommodate the bias circuit.

Semiconductors 82 and 8 4 are mounted within housings 28 and 30respectively, with fittings and mounting details substantially identicalto the mounting of diode 80 within housing 26 described above. Aplurality of diodes may be mounted in my invention in parallel as shownin FIGURE 3 with the advantage of increased power limiting capacity.Some frequency range at higher frequencies is lost with increasednumbers of diodes in the system. My invention operates through a usefulrange of power loads and a wide frequency band utilizing a single diodeelement; however, I have constructed models utilizing five diodes whichprovided greatly enhanced power limiting capability with only modestloss of response at higher frequencies.

Operation of my invention may be readily understood by reference to theschematic diagram in FIGURE 5. The gaseous electron discharge tube 46 ismounted at a point of maximum amplitude in a standing wave pattern whichis generated between the reactive posts 38 and 40 mounted within asection of coaxial wave guide. If the re-active posts are separated 1/2electrical wave length, the point of maximum amplitude will be midwaybetween the posts. A nonlinear diode 80 which is capacitive for lowpower incident RF waves and conductive for high power incident wlaves ismounted in the coaxial line 3A electrical wave length from the gas tube.A passive filter 86 having a limited band pass frequency is formed fromthe inductance of the posts 38 and 40 and the capacitance associatedwith the diode 80. The bias 'circuit 114 couples a small quantity ofenergy from the wave guide by means of the probe 116, rectiiies the biassignal by means of a diode 120 and connects to the RF grounded terminal80h of the diode 80. It is necessary to separate the RF ground from theDC ground to effectively utilize the bias circuit. Thus, a self-biasingeffect is obtained from the combination of elements which comprise myinvention.

It is to be understood that the electrical spacing may be varied fromthat described above provided an odd integral number of quarter wavelengths are maintained between the gaseous electron discharge tube andthe semiconductor.

A useful variation on the embodiment of my invention described above isillustrated in FIGURES 6 and 7 in which the bias circuit has beeneliminated for simplicity and reduction in cost and number of requiredcomponents. In the absence of the bias circuit the solid state elementmay be grounded without requiring separation of the RF and DC grounds.'I'he advantage is illustrated in FIGURE 6 which shows a cross sectionview of a housing 130 similar to that shown at 26 in connection withFIGURE l except as described below. The FIGURE 6 embodiment is in allrespects identical to that of the FIGURE 1 device except for thevariation in mounting of the solid state element. Referring now toFIGURE 6 a coaxial wave guide having an inner conductor 132 and an outerconductor 134 is provided with a threaded boss 136 that has a centralaperture and is integrally mounted externally to the outer conductor134. A transverse aperture 138 aligned with the boss 136 is 'provided inthe outer conductor 134. A socket contact 140 is brazed to the innerconductor inl 'gnment with the aperture 138.

A diode 142 having terminals 142a and 142b is mounted with the cathodeterminal inserted into the socket in contact with the inner conductor138. An enlarged terminal cap 144 is placed over the anode terminal 142bof the diode. A iianged dielectric sleeve 146` fits over the Idiodebody. A heat conduction ring 148 and a grounded heat sink block 150 areclamped to the terminal cap by means of a threaded cap 152. The mountingarrangement described above is readily visualized by reference to FIGURE6.

The operation of the solid state limiter illustrated in FIGURE 6 can bereadily understood by reference to FIGURE 7 wherein a gaseous electrondischarge tube is mounted at a point of maximum amplitude in a standingwave pattern generated by two spaced reactive shunt posts. The above isidentical with that arrangement shown in FIGURE 5 and described a'bove.The inductance of the posts combined with the capacitance of the diode142 form a passive band pass filter shown schematically at 160. Thediode 142 is mounted on odd integral quarter electrical wave lengthsfrom the gaseous electron discharge tube.

When high intensity RF waves impinge upon the cavity a standing wave ofsufficient intensity is generated to excite the ionizable gas Within thetube and an electron discharge results which shorts the w-ave guide andmomentarily prevents passage of vRF waves. Prior to discharge of thetube 'a spike of high intensity energy passes the plane of the tubeterminals and is propagated through the passive band pass filter 160, ifit falls within an appropriate frequency range. The presence of the highintensity RF waves switches the diode 142 from a capacitive to aconductive condition. The diode switching detunes the bandpass iilterwhich then acts to reflect a substantial portion of the high energyspike, and also conducts present residual enengy to ground. The absenceof the bias circuit does not alter the function of my invention exceptthat a slightly high intensity signal is required to switch the diodeinto a conductive condition then when the diode is otherwise biased intonear conduction before the arrival of high energy RF waves.

The foregoing specifications and descriptions are intended as merelyillustrative of my invention, the scope of which is set forth in thefollowing claims.

What is claimed is:

1. A device for limiting the power of high frequency electrical signalscomprising in combination -a section of coaxial wave guide having aninner conductor and an outer conductor, means within the wave guidewhich give rise to a discontinuity in impedance, a gaseous electrondischarge tube mounted at a spaced electrical distance from thediscontinuity in shunt between the inner and outer conductors, nonlinearsolid state means mounted in shunt between the inner and outerconductors at a spaced electrical distance from the tube; the solidstate means being essentially reactive at low power incidence andessentially conductive lat high power incidence; whereby electricalsignals of limited power only are passed lby the device, the power ofsuch electrical signals being limited when the gaseous electrondischarge tube fires, such power being further limited when thenonlinear means conducts between the inner conductor and the outerconductor, and bias means mounted within the wave guide to couple energyfrom the wave guide, said bi-as means Ibeing connected to said solidstate means whereby said solid state means may be biased into aconductive condition when the high frequency energy is coupled into thebias means.

2. A device for limiting the power of high frequency RF signalscomprising in combination a length of coaxial conductor having an innerand an outer conductor in coaxially spaced relationship; a plurality ofreactive impedance means mounted within the coaxial wave guide forgenerating a standing wave pattern upon incidence of RF signals, a twoterminal gaseous electron discharge tube mounted in shunt between thecoaxial inner and outer conductors at a spaced electrical distance fromthe impedance means; a variable reactance two terminal semiconductormeans mounted in shunt between the inner and outer coaxial conductors ata spaced electrical distance from the tube, and passive bias meansmounted Within the Wave guide to couple energy from the wave guide andbeing further connected to one terminal of the semiconductor meanswherewith the semiconductor means may be biased into a conductivecondition When RF energy is soupled into the bias means.

References Cited UNITED STATES PATENTS 3,131,365 4/1964 Hoover 333-13 X5 3,174,119 3/1965 Jones et al 333--13 X 3,249,899 5/ 1966 Broderick333-13 HERMAN KARL SAALBACH, Primary Examiner.

m R. F. HUNT, M. L. NUSSBAUM, Assistant Examiners.

1. A DEVICE FOR LIMITING THE POWER OF HIGH FREQUENCY ELECTRICAL SIGNALSCOMPRISING IN COMBINATION A SECTION OF COAXIAL WAVE GUIDE HAVING ANINNER CONDUCTOR AND AN OUTER CONDUCTOR, MEANS WITHIN THE WAVE GUIDEWHICH GIVE RISE TO A DISCONTINUITY IN IMPEDANCE, A GASEOUS ELECTRONDISCHARGE TUBE MOUNTED AT A SPACED ELECTRICAL DISTANCE FROM THEDISCONTINUITY IN SHUNT BETWEEN THE INNER AND OUTER CONDUCTORS, NONLINEARSOLID STATE MEANS MOUNTED IN SHUNT BETWEEN THE INNER AND OUTERCONDUCTORS AT A SPACED ELECTRICAL DISTANCE FROM THE TUBE; THE SOLIDSTATE MEANS BEING ESSENTIALLY REACTIVE AT LOW POWER INCIDENCE ANDESSENTIALLY CONDUCTIVE AT HIGH POWER INCIDENCE; WHEREBY ELECTRICALSIGNALS OF LIMITED POWER ONLY ARE PASSED BY THE DEVICE, THE POWER OFSUCH ELECTRICAL SIGNALS BEING LIMITED WHEN THE GASEOUS ELECTRONDISCHARGE TUBE FIRES, SUCH POWER BEING FURTHER LIMITED WHEN THENONLINEAR MEANS CONDUCTS BETWEEN THE INNER CONDUCTOR AND THE OUTERCONDUCTOR, AND BIAS MENS MOUNTED WITHIN THE WAVE GUIDE TO COUPLE ENERGYFROM THE WAVE GUIDE, SAID BIAS MEANS BEING CONNECTED TO SAID SOLID STATEMEANS WHEREBY SAID SOLID STATE MEANS MAY BE BIASED INTO A CONDUCTIVECONDITION WHEN THE HIGH FREQUENCY ENERGY IS COUPLED INTO THE BIAS MEANS.